CODEWITHSUNDEEP
c++bit-manipulationbitwise-operators
user997112
user997112

Reputation: 30605

How to get all ones or all zeros from boolean result?

I (stupidly) thought if I took the boolean result of true, cast to an int and left-shifted it I would end up with the LSB repeated at every bit, obviously not!

If I had a boolean result and I wanted to convert this to all ones for true and all zeros for false, what would be the cheapest way (computationally) to do this?

bool result = x == y;

unsigned int x = 0;

//x becomes all ones when result is true
//x becomes all zeros when result is false

Upvotes: 3

Views: 503

Answers (4)

user1804599
user1804599

Reputation:

A solution that is more readable IMO:

unsigned int set_or_unset_all_bits(bool comp) {
    return comp ? ~0u : 0;
}

Upvotes: 7

Ortwin Angermeier
Ortwin Angermeier

Reputation: 6183

Maybe something like that:

#include <limits>
#include <stdint.h>

...

uint32_t x, y;

// init x and y with some values

if(x == y) {
  // all bits to 1
  x = std::numeric_limits<uint32_t>::max();
} else {
  // all bits to 0
  x = 0;  
}

Upvotes: 1

user3701522
user3701522

Reputation: 307

How about something like

int main()
{
      unsigned int x, y;
      bool b = x == y;
      x = b ? std::numeric_limits<size_t>::max() : 0;
}

Upvotes: 1

Paul R
Paul R

Reputation: 212939

Like this, perhaps:

bool result = x == y;

unsigned int z = -result;

Upvotes: 9

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